1. Field of the Invention
The present invention relates to the determination of the fluid level in the tubing-casing annulus of an oil well, gas well, or water well on a real time basis utilizing equipment which may be located at or near the ground surface. The present invention provides for the rapid, accurate, and relatively easy determination of the fluid level in the tubing-casing annulus through the imposition of a pressure wave—specifically a shock wave in contrast to the known systems which utilize acoustic waves. Embodiments of the present invention may also provide a history of the fluid levels and performance history of the artificial lift equipment. The present invention further provides the necessary input for a motor control means, such as motor starter or variable frequency controller connected to an electrical motor operating an artificial lift system or pumping system, to achieve and maintain the optimal production rate for an oil well or the injection rate of a water injection well. The integration of a real-time fluid level detection device together with a motor control means, such as a variable frequency controller, allows the optimization of well bore inflow with the well outflow provided by the artificial lift system, such that the outflow provided by the artificial lift equipment efficiently corresponds to the inflow of the reservoir.
2. Description of Practices in the Art
It is known that fluids are replenished into a particular well bore at different rates even in the same formation or well field. Such replenishment is impacted by, among other things, the section (i.e., length) of reservoir exposed to perforations or slots, any formation damage adjacent to the well bore, and/or the extent of reservoir heterogeneities adjacent to the well bore. Moreover, fluid replenishment into a particular well bore may change over time as a result of changes in reservoir properties resulting from cumulative production, stimulation or reservoir management practices, such as pressure maintenance. When a fluid reservoir is initially produced, there may be sufficient reservoir energy to produce the fluids to the ground surface, i.e., the pressure of the fluid reservoir is greater than the hydrostatic pressure exerted by a fluid column which extends from the ground surface to the depth of the reservoir. However, particularly in the case of solution gas drive reservoirs, the reservoir energy usually decreases as the reservoir is depleted to where the reservoir pressure is eventually less than the hydrostatic pressure exerted by a column produced fluid within the well bore.
When the reservoir energy is not sufficient for the reservoir fluids to flow to the surface some form of artificial lift system is required. Such artificial lift systems typically utilize some type of subsurface pump which is installed at the approximate depth of the producing reservoir. These artificial lift systems include positive displacement pumps, centrifugal pumps, jet pumps, piston pumps, and progressive cavity pumps.
One commonly known artificial lift system utilizes a plurality of rods connected in an end-to-end configuration forming a “rod string.” The rod string is set inside a plurality of tubing joints which are likewise connected in an end-to-end configuration forming the “tubing string,” with the reservoir fluids primarily produced up the tubing string in the annulus between the rod string and the tubing string. The rod string is utilized to operate a pump set at the bottom of the tubing string. The most commonly used subsurface pump is a positive displacement pump having a plunger which reciprocates up and down within a barrel, where the plunger is connected to the rod string and the rod string is reciprocated by a pumping unit set at the ground surface. Another type of subsurface pump, a progressive cavity pump, has a rotor which is rotated within a stator by the rod string, where the rod string is rotated at the ground surface by an electrical motor coupled to a gear reducer. Electric submersible pumps are also used, where the motor is located downhole and typically coupled directly to a centrifugal pump. In piston pump installations, a surface pump injects a power fluid into the well which operates a down hole piston pump. In jet pump operations, a surface pump injects a power fluid which flows through a downhole venturi to create the required lift to produce the reservoir fluids.
The starting and stopping of each of the above-described pump systems may be controlled by a signal provided to the motor starter of each prime mover operating the downhole pump, such that the pump capacity is simply adjusted by controlling the run time of the downhole pump, which is done with systems which run on timers. Another means of adjusting the capacities of each of these artificial lift systems is by adjusting the pump speed by adjusting the speed of the motor operating the pump. Controlling the motor speed may be accomplished by utilizing a variable frequency drive.
With each subsurface pumping system, a dynamic equilibrium is reached where the inflow rate of the reservoir fluids and the outflow rate of the fluids generated by the artificial lift system are essentially equivalent (excepting free gas which is not produced by the subsurface pumping system but rather produced as a separate phase, typically through the casing-tubing annulus). However, the inflow rate from the reservoir into the well bore will be impacted by any backpressure maintained on the reservoir inside the wellbore. Such backpressure results from any fluid column in the wellbore above the producing zone, in combination with any pressure applied at the surface at the casing-tubing annulus, such as any pressure imposed by a gas collection system.
Ideally, the backpressure applied at the surface and the fluid level within the tubing-casing annulus are each maintained at minimal levels, which maximizes the pressure differential from the reservoir into the well bore. Maximizing this pressure differential, in turn, maximizes fluid flow or inflow into the well bore. However, achieving this maximum inflow requires a corresponding matching outflow produced by the artificial lift system to reach a dynamic equilibrium. In other words, to achieve maximum production from a well, the well outflow rate generated by the artificial lift system must match the maximum inflow rate produced from the reservoir to minimize the backpressure exerted by any fluid column standing within the well bore above the producing zone.
The preceding discussion suggests that to maximize production, the subsurface pump should be run so as to keep the level in the well bore as low as possible. However, this option may be less than ideal because if the outflow produced by the artificial lift equipment exceeds the inflow, i.e, the pumping rate of the artificial lift equipment exceeds the rate of flow into the wellbore from the reservoir, several negative results may occur. First, running the pump constantly or at too great a speed may be inefficient because, at times, the well may be “pumped off” leaving little fluid in the well bore to be pumped, resulting in wasted energy. Second, running pumping equipment when a well is in a pumped off condition can damage the equipment, resulting in costly repairs. Third, paraffin build up is more pronounced when a well is allowed to pump dry. In a pumped off condition gases are drawn into the well bore, which expand and cool. As the gases cool, paraffin build up is promoted as the hydrocarbons begin to plate out on the surfaces of the well bore.
Achieving equilibrium between inflow and outflow is further complicated by changing conditions within the reservoir, which result in changes in inflow performance. Such changes may result from, among other things, the initiation or suspension of a reservoir pressure maintenance program utilizing either gas or water injection, stimulating the well to remove reservoir damage near the well bore, or stimulating injection wells to increase injection rates. The reservoir conditions may also be impacted by the addition of new wells producing from the reservoir or changing production rates in existing wells which produce from the same reservoir. Thus, matching inflow performance of the reservoir with the outflow of the artificial lift system can present a moving target and an artificial lift system which maintains a constant outflow is not a preferred solution for a well subject to changes in its inflow performance.
A variety of methods are known for adjusting the outflow performance of an artificial lift system in accord with the inflow performance. Systems which utilize reciprocating rod pumps may have adjustments made to the outflow performance by changing the speed of rod reciprocation, changing the length of the pump stroke, or changing the diameter of the subsurface pump. Changing pumping speed and pump stroke for rod pumped wells usually can be accomplished by making adjustments in surface equipment, however changing the pump diameter requires pulling the rod string, pump, and often the tubing string. Changing the speed of rod reciprocation can be done by causing the surface pumping unit to run faster by either changing the sheave size between the prime mover and gear box, or by changing the operational speed of the pumping unit motor. Changing the sheave size requires the shutting down of the pumping unit and can be an involved process requiring a construction crew Likewise, stroke adjustments may be made at the surface so long as the subsurface pump has sufficient length to accommodate any increases in stroke length. Stroke length changes also normally require the services of a construction crew and the shutting down of the pumping unit.
Changing the operational speed of the motor may be accomplished through the use of a variable speed drive unit, or variable frequency drive (“VFD”). If a VFD is combined with a processing unit, various input parameters, including observed fluid levels, may be utilized to arrive at a pumping speed, and thus a particular outflow capacity, which is in dynamic equilibrium with the reservoir inflow performance. Such systems may be used not only with reciprocating rod pumps, but also with rod-operated progressive cavity pumps, downhole submersible pumps and other pumps which are operated by electric motors.
U.S. Pat. No. 6,085,836, invented by, among others, D. R. Hill, one of the present inventors, proposed an initial solution to the problem of reaching dynamic equilibrium between reservoir inflow performance and the outflow performance of the artificial lift equipment. The '836 patent is incorporated herein by reference. The '836 patent discloses a method of determining the well fluid level for purposes of adjusting the subsurface pumping time, including controlling pumping time with timers. It is known to use timers to control the pump duty cycle. A timer may be programmed to run the well nearly perfectly if one could determine the duration of the on cycle and off cycle which maintains a dynamic equilibrium between the inflow to the well bore and the outflow generated by the artificial lift equipment.
If real time fluid level information can be obtained, deciding when or how fast to run the pump is relatively straightforward and production can be optimized. Real time fluid level determinations, particularly for deep well systems, have been realized by the implementation of downhole instrumentation such as load cells, transducers or similar devices which acquire downhole pressures (thus fluid levels) and transmit the information to the surface via various means. Unfortunately, these real time downhole systems may be costly and complex to install, unreliable in operation, and costly to repair or service, typically requiring the removal of the rods and production tubing with a production rig or work-over unit. Although the implementation details will not be discussed here, it is worth noting that these systems, when operating correctly, have proven that significant gains in well production are available when control strategies applying real time fluid level measurement are utilized.
As an alternative to systems which measure downhole pressure with downhole devices, are those systems which utilize acoustic energy to ascertain the depth of the fluid level by generating an acoustic wave at the surface and detecting the return signal to calculate the depth to fluid. One such system uses a one-shot measurement. The one-shot measurement will use a sonic event, such as firing a shotgun shell, to generate the acoustic signal. Another system utilizes charges from a nitrogen tank to generate sonic events. However, in either of the foregoing systems the production of the well must usually be shut down before initiating the sonic event and monitoring the corresponding return signals.
As an alternative to the one-shot measurement systems are those which are programmed to provide periodic acoustic signals, and which do so while the surface equipment is in operation. Such a system is described in U.S. Pat. No. 8,281,853, of which an inventor of the present invention, D. R. Hill, is an inventor. The system of the '853 patent may utilize produced gas from the well to generate the acoustic signal. These systems have provided a good solution for optimizing well production by real time adjustment of the well outflow in accord with changes in flow into the wellbore.
Improved accuracy in the fluid level measurement provides great advantage in matching the outflow of the artificial lift equipment with the reservoir inflow. Moreover, greater accuracy in a series of fluid level determinations combined with other monitored production parameters, such as real time production rates, flowing pressures and temperatures, etc., allows greater accuracy in determining other operational parameters such as determination of real time fluid densities, or ascertaining with greater accuracy the real time impact on one well as a result of changes in the production/injection rates of adjacent wells. Such information may be utilized efficient reservoir management, where the production rates and injection rates in a particular reservoir may be optimized according to the observed parameters by utilizing motor controllers, such as variable frequency drives on artificial lift equipment and injection pumps. However, one obstacle to obtaining accurate readings is the presence of noise in the well caused by the mechanical operation of the well equipment and by the various noises produced by the flow of fluids, and sometimes sand, into the wellbore.